This application claims priority under 35 USC 119 of Japanese Patent Application No. 2001-142564 filed in JPO on May 14, 2001, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus which can be applied to printers and facsimile machines, and relates to an image forming method.
2. Description of the Related Art
The electrophotographic image forming apparatus conventionally employs a reversal development method. According to the reversal development method, an organic photo conductor (OCP) film is formed on a drum functioning as an image bearing body (for, example, a photoconductive body), and a latent image is formed at a part of the photoconductive body which has been exposed to light, and toner is attached to the latent image, thereby forming a developed image. In this kind of image forming apparatus, sensitivity of the organic photo conductor (OPC) film becomes low in the environmental condition where a temperature and humidity are low. Furthermore, in such environmental condition, potential of the electrostatic latent image does not rise sufficiently, the density of the toner image formed on the image bearing body is decreased, and as a result, the density of the image transferred to the recording paper is also decreased. Japanese Patent Application, Laid Open Publication No. 8-171147 discloses an image forming apparatus for controlling the exposure time of the image bearing body based on the environmental temperature detected by a provided temperature sensor.
As described above, the problem that the density of the image transferred to the recording paper is decreased due to a surrounding temperature drop can be relieved by such an image forming apparatus for controlling the exposure time of the image bearing body based on an environmental temperature. However, there is another problem that the temperature sensor must be installed. Meanwhile, in the environmental condition where an temperature and humidity are high, the organic photo conductor is highly sensitive, and the potential of the electrostatic latent image rises sufficiently. However, since the transfer efficiency of the image from the image bearing body to the recording paper is deteriorated at high temperatures and humidity, the density of the image recorded on the recording paper is lowered as a result.
The present invention was devised in focusing on the above problems, and it is an object of the present invention to provide an image forming apparatus which can prevent decreasing of the density of the image transferred to the recording paper which is caused due to a drop and rise in temperature even if the temperature sensor is not specially provided.
According to one aspect of the present invention, there is provided an image forming apparatus including an exposure unit for exposing an image bearing body to light so as to form an electrostatic latent image on the image bearing body, a developer for developing the electrostatic latent image on the image bearing body, and a transferring member for transferring the image developed on the image bearing body to the paper. This image forming apparatus determines a transfer bias to be applied to the transferring member and exposure energy of the exposure unit based on the current value generated at the time a test voltage is applied to the transferring member.
According to another aspect of the present invention, the image forming apparatus determines the transfer bias to be applied to the transferring member, and controls the exposure energy of the exposure unit, in accordance with the current value which is generated at the time the test voltage is applied to the transferring member. Preferably, the image forming apparatus increases the exposure energy when the current value is low due to a low surrounding temperature, or when the current value is high due to a high surrounding temperature, thereby preventing decreasing of the density of toner image which is transferred to the paper. Furthermore, the image forming apparatus preferably includes a table for determining the exposure energy, based on a value of an electric current generated at the time the test voltage is applied to the transferring member. In addition, the image forming apparatus preferably includes a transfer bias applying unit for applying the transfer bias to the transferring member.
Preferably, the exposure energy is controlled by controlling a period of time for which the exposure is being carried out, or is controlled by controlling intensity of light to be used for exposing the image bearing body.
Additional objects, aspects, benefits and advantages of the present invention will become apparent to those skilled in the art to which the present invention pertains from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings.